Photographic paper having reduced processing curl



United States Patento F PHOTOGRAPHIC PAPER HAVING REDUCED PROCESSING CURL Application November 8, 1954, Serial No. 467,631

6 Claims. (Cl. 96-8S) No Drawing.

This invention concerns a method of treating photographic paper to eliminate or reduce the processing curl.

Early in the history of photographic paper manufacture, a slow rate of drying was used following coating with photographic. emulsion. The curling characteristics of the paper thus prepared were not too unfavorable, particularly when the paper was stored in a place where there was not a great deal of variation in the relative humidity of the air in the place of storage. In recent years, however, the tendency has been to speed up the rate of drying of the emulsion coated paper, which has involved the use. of elevated temperatures in those drying operations.

The cellulose fiber in the paper base of photographic paper and the gelatin in the emulsion layer thereof are both hygroscopic. They expand as they absorb water vapor at high humidity and contract as they desorb water vapor at low humidity. The rates of expansion and contraction of the two materials are not necessarily the same under a given set of conditions so that the stresses and strains set up in photographic paper often result in curl. This is especially true in the case of light-weight paper stocks such as those having a weight of 8-15 pounds per thousand square feet. When such paper is coated with an emulsion, often the stress in the emulsion layer is sufficiently great to result in face curl, or in other words, the curling of the paper towards the emulsion-coated side.

When wet photographic emulsion is applied to dry paper, some of the moisture from the emulsion penetrates the paper resulting in expansion of the paper base, while loss of water by the emulsion causes its contraction. Thus, the expansion of the paper and the contraction of the emulsion at that time sets up a balance of forces somewhat diiferent from that obtained in the paper and the emulsion immediately prior to the absorption of water by the paper.

To avoid internal crackingof certain emulsions, sometimes the photographic emulsion is coated on the wire'side of paper stock which provides a denser surtace than the other side. As a result, the paper may curl towards the back (non-coated) side during processing when the processing solutions penetrate the paper and emulsion. The curl actually starts out as a face curl (towards the coated side) immediately upon immersion in the developer and then reverses to a back curl (towards the uncoated side) after about half the development time is up. At the time of development, the curl may be more extreme. It may become more pronounced during the balance of the processing cycle especially when a water rinse is used in place of short stop and when the wash water after fixing is on the alkaline side. In addition, the processing curl may tend to become more noticeable as the sensitized paper ages, usually reaching the worst point at about 6 months age.

I have found that I can improve this process curl, especially with regard to the aging factor by coating a light size coat of a non-continuous film of polystyrene under the baryta on a wire side coated paper.

One object of my invention is to provide a photographic paper with reduced process curl. Another object is to provide a photographic paper coated on the wire side of the paper stock, which is relatively free from processing curl. An additional object is to provide a method for treating photographic paper, so that the paper will withstand wide variations in relative humidity without any appreciable curl.

The above objects are obtained by coating the wire side of photographic paper with a polystyrene resin dispersion in water containing about 1015% solids. The coating is applied at about 2.5-4 grams per square meter and heat treated at a paper surface temperature of 275- 320 F. for 5-10- seconds. Following the polystyrene coating, the paper is coated with a baryta-adhesive layer 7 which is dried. A light-sensitive silver halide emulsion is superimposed over the baryta coating.

The following examples illustrate but are not intended to limit my invention.

Example l.--A roll of paper stock was coated'on the wire side with three grams per square meter of a straightv Equilibrium curl.

Polystyrene coated paperim 2.O

Control sample, withoutthe polystyrene coating,

but with the'sam'e silver bromide cmulsion 8.7.

Equilibrium curl. is the value obtained by measuring the change in distance across the arc of the curl from one i edge of a 10 cm. disc of paper to the edge diametrically.

opposite. Therefore, if the paper were absolutely flat during processing there would be no change in this distance. However, when the paper curls this distance grows shorter. A value of 2.0, therefore, would denote that the paper has curled slightly, and the distance has shortened by 2 cm. The value of 8.7 cm. denotes a more pronounced curl, the distance being shortened by 8.7 cm.

or that the edges areonly 1.3 cm. apart. A negative sign denotes cur'l towards the back, or uncoated side; The following results indicate the criticality of the weight of coatingapplied in regulating curl. .The 2.54 grams per square meter coverage is quite critical as is shown by the following results.

Coated paper samples: Equilibrium curl 1 a. 1.5 grams per square meter -5.5

b. 5.0 grams per square meter -7.4 c. 3.0 grams per square meter 3.3 d. Uncoated sample 9.1

1 10 minutes in processing solutions.

Microscopic examination showed that the resin particles in the 3.0 grams per square meter coat were not completely fused, and therefore, the film was not continuous but full of very minute crevices which are of such a nature as not to permit very rapid solution penetration. The 1.5 gram coat was too thin to ofier suflicient solution penetration. The cracks in the 5.0 clear coat had grown to suflicient size to appear as dry, cracked mud and to permit rapid solution penetration.

The heat treatment of the size coat described in Ex ample I is essential to obtain partial resin particle fusion i atented Mar. 5, 1957 The coated stock was then heat treated.

with-resulting optimum-curl correction as is shown by the= following: data:

Coated Paper Samples: Equilibrium curl a..3. grams.- per sqpare. meter. heated. at-

275.."30Q? 117., w ....n -32 11,, .3, grams-per. square meter.- dri'edat 1:105 B- nly. a. e. Untreated: control,. "PM... -7-.L

A: temperature; of about 50060(I F: is necessary" for complete resin-particle fusion; It is obvious that the paper itself would not withstand thesetemperatures. The coatingiapplied using 3- grams persquare meter and heat; treated? at, at temperature ofi 275 320 B. alsoofferedgood resistance toaging characteristics as fol lows Equilibrium curl 0., 3T grams. per square. meters, sensitized. with. a

silver.hromideemul ionhfteshu- 2:0 Accelerated aging; ..r .s.. 3..3 .6. ay incubation. 120) 35%) relative.

humidity 1'.4.

b. Uncoatedcontrol sensitized withsilver bromide.

emulsion, fresh l 5..l1 Accelerated. aging, ,n -9.L

5f dflysincubation. at. 120 F.,. 5% relative humidity 10.5

latex: sized stock. The" paper lay quite flat or had very' slight face curl uponprolonged-developing orin a' water rinse after developing: The curlwascomparable'to that" of face. side coated 'stock.

Azcoating: of polystyrene resin latex may lie-appliedumrneath; the -.bar.ytas layer; on either the fact-(aside or qtwireasidc; ofi thezzpaper: depending. upon the, necessity zeurlzcorre tiont. Inasmuch;as:photographic-papencurl is usually due tothe. shrinkage-oithe emulsion: omdrying ...the coa ingwouldrhe appliedmnder thabaty coat-a ingmver: which he photogr phic. emulsion-t is sunerims posed- Almost. all: resin. dispersions when: coated. on.

p g r. in which the, percentage, of theresin, dispersion. is

bigger than 60% have beenfaundfltotattotd.poor. adhesive.

surfaces for other coatings such as gelatinous baryta coatings,v However I have found thatusing a: clnating of about 2.54 grams per square meter of polystyrene heat-treated at a tempera urerof, 275-320?- E-thatiadiscontinuous coating is obtained which; permits the gelatinous baryta coating to adhere inspite oftherigid'requirements of the photographic industry, particularly" involving processing steps wherein the paper is immersed in alkaline solutions" or" in acid stop Baths followed hy heat drying.

It will be obvious to those skilled in the art that various paper stocks may be used and that the baryta-gelatin coating may be SUbStllIUteEZbYQOIhCI' coatings containing mineral. fillers andadhesivessuch as.easein,,resin latices and the like. Care-must'he'taken', of; course, thatthose mineral filler compositions be compatible both with the polystyrene; and; with the photographic, emulsion:

I claim:

1. In a process for providing a photographic paper having at light=sensitive Sill/B12 halide. photographic.v emulsion coated over a barytaiayeron; the wire side of the paper, the step of first coating the paper under the baryta layer with 26-4 grams per square meter of an aqueous polystyrene resin latex and heat treating at a temperature of, 275-3207..

22 Aphotographicpaper containingthree coatings over. the paper Base, the first. comprising 2.5.-4.' grams. per square meter. oflan. aqueouspolystyrene resinlatex partly fused, so that. a discontinuoussurface. is. formed. the second"comprisingabaryta layer and the. third. comprising azlight-sensitive silver. halide. photographic emulsion.

3.; A photographic paper containing. three. layers. on.

thewire sideofjthe paper hase thefi'rst comprising2.5.4 grams per. square meter 01:" av polystyrene. resin, partly fused so. that a, discnntinunus. surface. is. formed, the secondfcnmprising'a harytaadhesive layer, andthe third.

comprising; a light-sensitivev silver. halide photographic emlJI lQm.

4;.Ii1 a photographic paper: having a .lighhsensitive. silyer. halide photographic, emulsion coated over aliaryta.

theZ wiresideof; thepaper under. the. barytalayer of 2.5-4

gramsper. square meter. ofi'pol'ystyrene.resinpartly fused so that. a discontinuous, surface. is formed..

6;,A process. for providing. a. photographic paper havihg improved; processing curl, comprising coating the. paper.- with; 2.5.4 grams, pen square. meter; of. a.v polystyrene. resin. latex heat, treating at a paper surface temperatuneoi 275.3v20 E. for 5.-10.s.econds,, coating with.

a haryta-adhesive; coatingpomposition, drying, and: super.

imposing, over, he. dried. haryta layer at light-sensitive.

silver. halide emulsion.

Refereneessfiitedzin thezfilaofrthisapatent UNITED STATES PATENTS Galleyr Jan. 9; I945- O'PHER REFERENCES Styrene. by. Boundy-Boyer, published by Reinhold,.

New Yank, N. Y.,. 1952.; pages. 857-860. 

1. IN A PROCESS FOR PROVIDING A PHOTOGRAPHIC PAPER HAVING A LIGHT-SENSITIVE SILVER HALIDE PHOTOGRAPHIC EMULSION COATED OVER A BARYTA LAYER ON THE WIRE SIDE OF THE PAPER, THE STEP OF FIRST COATING THE PAPER UNDER THE BARYTA LAYER WITH 2,5-4 GRAMS PER SQUARE METER OF AN AQUEOUS POLYSTYRENE RESIN LATEX AND HEAT TREATING AT A TEMPERATURE OF 275-320*. 